The present invention relates to paper machines.
As is well known, in such machines the pulp suspension or pulp stock from which the paper is manufactured is delivered by a pipe system to a headbox, and the present invention is primarily concerned with apparatus for damping pressure fluctuations in the stock suspension which flows along the interior of the pipe system to the headbox.
As is well known, the pulp suspension flowing in the pipe system to the headbox is exposed to pressure fluctuations, for a number of reasons, and these fluctuations if transmitted all the way to the lip slice of the headbox will result in lack of uniformity in the paper which is manufactured. The situation with respect to such pressure fluctuations is ideal when throughout the entire lip slice of the headbox there is a suspension flow in which the quantity of dry matter suspended is uniform while the velocity of flow is constant. If this flow is uniform over the entire width of the slice but varies with passage of time, then in the longitudinal direction of the paper machine there will be variations in the dry weight of the paper.
If the pulp suspension flow is maintained constant with respect to time but is different at different locations along the width of the paper machine, then there will be a transverse variation of weight in the dry content of the paper. This type of variation is not damped by way of the present invention, nor by way of any other known damping systems acting at the pipe system which delivers the stock to the headbox, but transverse equalization can be taken care of by proper adjustment of the lip slice through suitable known spindles which are available for providing a fine adjustment of the depth of the lip slice across the width thereof.
If the pulp stock flow, when measured simultaneously at the entire aperture of the lip slice, is independent with respect to time and in addition, at any point across the width of the paper machine, is on the average equal over a relatively long interval but different from one point of time to another point of time, then there will be produced in the paper, across the breadth of the machine, randomly situated heavier and lighter areas, or so-called residual variation. Such variations are caused first as a result of turbulence vortices produced in the headbox and acting on the output flow rate and secondly by a small-scale non-uniform distribution of the dry matter in the pulp suspension.
This latter turbulence cannot be damped but is influenced by the particular design of the headbox. On the other hand, the small-scale dry matter distribution in the pulp suspension will indeed be equalized by way of the present invention in a manner which is superior to or at least equal in economy and efficiency to the best previously known designs.
The variation in the dry weight of the paper longitudinally, in the direction of the machine, is primarily caused by variation in the flow rate which occurs in the stock input to the headbox by way of the pipe system. Secondly, such variations are caused by pressure waves which are always present in the pipe system while being propagated with the velocity of sound, the latter variations being converted at the aperture of the slice into variations in the kinetic energy of the jet. In the third place these variations are caused by large-scale consistency variations in the stock supply pipe.
Thus, in summary, it is to be observed that the input fluctuation signals with respect to which the present invention is concerned are the dynamic pressure variations at the lip slice, while the output fluctuation signals are the variations in the hydrostatic pressure in the pipe system, variation of the pressure at the supply pump, variation in the pressure drop occurring in the flow, pressure pulses due to vibrations which are transmitted to the pipe system through the supports thereof, and pressure variations resulting from turbulence vortices particularly at valves, at bends in the pipe system, etc.
It has been found in practice that the different pressure fluctuation signals each have their own characteristic frequency spectrum which often is fairly wide. However, the pressure fluctuation signals of pumps, for example, are characterized by clearly observable peaks at frequencies consistent with the speed of rotation of the pump and its multiples and subharmonics.
Paper machine headboxes, as shown in the prior art, may be divided into three main groups, namely, (a) headboxes provided with an air cushion situated directly in the headbox, or so-called air cushion headboxes, (b) hydraulic headboxes provided with an air cushion separate from the headbox itself, wherein the air tanks are located either in the approach pipe system of the pulp suspension in advance of the distribution header or after the distribution header, and (c) hydraulic headboxes which have no air cushions at all.
By utilizing air cushions in connection with the headbox, an attempt is made to equalize pressure variations occurring in the pulp suspension flow in advance of the lip slice, such pressure variations originating either in the stock system preceding the headbox or in the headbox itself.
In the air cushion headbox according to group (a), the damping of the pressure variations, during the passage of time, is in most cases highly efficient because in these cases the surface area of the flowing stock which is in contact with the air cushion is comparatively large, while the depth of the stock flow, measured at right angles to the flow direction, is relatively small. Such headboxes also have the advantage that in them the air cushion usually extends up to a location which is quite close to the discharge lip or slice of the headbox, so that at the region between the location where the air cushion acts and the slice the possibility of generation of new pressure fluctuations is at a minimum.
However, in spite of the above advantages of the group (a) type of headboxes, this type has in recent times been replaced in modern, fast paper machines by hydraulic or fully hydraulic headboxes as set forth above in groups (b) and (c). The reason for this is the easier accommodation of the latter types of headboxes in connection with the more modern twin-wire formers and on the other hand the lower manufacturing costs involved with groups (b) and (c). The greater turbulence of the stock jet discharging from the slice and the more favorable intensity distribution therein, together with the superior homogeneity of the stock resulting therefrom, have also promoted the use of these hydraulic headboxes. However, as against the above advantages thereof, hydraulic headboxes have displayed problems arising from the above pressure fluctuations. It has very often been necessary to provide a headbox originally intended to be fully hydraulic with one or more separate air tanks intended to replace the air cushion of an air cushion headbox. With respect to the location of these separate air tanks, various design solutions have been provided, and in some of them the air tanks have been connected to the pulp stock pipe system in advance of the headbox. In other known designs, the air tanks are situated above the headbox itself, being joined thereto by connecting pipes or by a connecting duct communicating with the upper part of the headbox.
Constructions of this latter type, however, have the drawback that in the air tank above the headbox the depth of the free liquid over the central axis of the liquid stream is relatively large, or the communicating pipes from the headbox to the air tank must be dimensioned so as to be relatively narrow as compared with the cross section of the main flow passage. In both of these cases the damping characteristics are substantially impaired, as compared with the pressure variation damping capability of the standard air cushion headbox.